In recent years, liquid crystal display apparatuses are widely used in a variety of fields, such as AV (Audio Visual) and OA (Office Automation) fields. In particular, LCD apparatuses of the passive type, which use TN (Twisted Nematic) or STN (Super Twisted Nematic) liquid crystal, are installed in low-end products. Further, LCD apparatuses of the active-matrix driving system are installed in high-end products. The LCD apparatuses of the active-matrix driving system use, as switching elements, TFTs (Thin Film Transistors), that is, three-terminal elements, MIM (Metal Insulator Metal) elements, that is, two-terminal non-linear elements (hereinafter referred to as two-terminal elements), or others.
The LCD apparatuses of the active-matrix driving system have features that are superior to those of CRTs (Cathode Ray Tubes) in color reproducibility, thinness, light-weight, and low power consumption, and the application of these displays has been rapidly expanding. However, LCD apparatuses using three-terminal elements such as TFTs as switching elements require thin-film forming processes and photolithography processes of 6-8 times or more during production of an LCD apparatus, resulting in high production costs. In contrast, LCD apparatuses using two-terminal elements such as MIM elements as switching elements, requiring less processes, are less expensive to produce compared with those using three-terminal elements, though display quality of the same is inferior to that of LCD apparatuses using three-terminal elements. In addition, LCD apparatuses using two-terminal elements also exhibit superior display quality compared with those of the passive type. Therefore, the use of the LCD apparatuses using two-terminal elements has been rapidly developing.
Furthermore, a voltage averaging driving method, which is a driving method for LCD apparatuses of the passive type, has an advantage that it can be adopted to LCD apparatuses using two-terminal elements. Therefore, the LCD apparatuses using two-terminal elements can realize high contrast and homogeneity in display.
As shown in FIG. 6, an LCD apparatus using the two-terminal elements has, for example, a display panel 61, a data electrode driving circuit 62, a scanning electrode driving circuit 63, and a control unit 64.
The display panel 61, as is the case with a usual LCD apparatus, includes data electrode lines X1 through Xn and scanning electrode lines Y1 through Ym, which are disposed in a matrix form. As shown in FIG. 7, a liquid crystal element 71 and a two-terminal element 72 such as MIM elements, which are connected in series with each other, are installed at each pixel, the pixels being formed by intersection of the data electrode lines X1 through Xn and the scanning electrode lines Y1 through Ym.
The data electrode driving circuit 62, which is usually composed of a shift resistor, a latch circuit, and an analog switch, etc. (not shown), is arranged so as to apply fixed voltages which correspond to display data, to the data electrode lines X1 through Xn provided in the display panel 61.
The scanning electrode driving circuit 63, which is usually composed of a liquid crystal driving power generating circuit, a shift register, and an analog switch, etc. (not shown), is arranged so as to apply fixed voltages in a line-sequential manner to the scanning electrode lines Y1 through Ym provided in the display panel 61.
The control unit 64 is equipped with a liquid crystal driving signal control section 65 and a liquid crystal driving voltage generating section 66. The control unit 64 is arranged so as to send control signals and liquid crystal driving voltages V.sub.0 through V.sub.5 to the data electrode driving circuit 62 and the scanning electrode driving circuit 63, so that inputted information is displayed in accordance with display data supplied from an input signal line 67.
The liquid crystal driving voltage generating section 66, as shown in FIG. 8, is arranged so as to produce electric potentials at 6 different levels, namely, electric potentials V.sub.0 through V.sub.5, using a voltage (V.sub.EE) supplied by a liquid crystal driving power source 81, with a split resistor 82 and an operational amplifier (hereinafter referred to as OP amplifier) 83. The electric potentials V.sub.0 through V.sub.5 are sent as liquid crystal driving voltages V.sub.0 through V.sub.5 to voltage applying lines V.sub.0 !, V.sub.1 !, V.sub.2 !, V.sub.3 !, V.sub.4 !, and V.sub.5 !.
Among the voltage applying lines V.sub.0 !, V.sub.1 !, V.sub.2 !, V.sub.3 !, V.sub.4 !, and V.sub.5 !, as shown in FIG. 6, those V.sub.0 !, V.sub.2 !, V.sub.3 !, and V.sub.5 ! are arranged so as to supply voltages to the data electrode driving circuit 62, while the voltage applying lines V.sub.0 !, V.sub.1 !, V.sub.4 !, and V.sub.5 ! are arranged so as to supply voltages to the scanning electrode driving circuit 63.
The liquid crystal driving signal control section 65 is arranged so as to transmit, as control signals, a latch pulse LP (see FIG. 9(a)) and an AC conversion signal M (see FIG. 9(b)) to the data electrode driving circuit 62 and the scanning electrode driving circuit In accordance with the latch pulse LP as a control signal and the AC conversion signal M, fixed voltages (selected among the 6 liquid crystal driving voltages V.sub.0 through V.sub.5) are respectively applied to the scanning electrode lines Y1 through Ym and the data electrode lines X1 through Xn of the display panel 61 by the scanning electrode driving circuit 63 and the data electrode driving circuit 62.
For example, in the case where voltages represented by waveforms in FIGS. 9(c) and 9(d) are applied to a scanning electrode line Y1 and a data electrode line X1 respectively, a voltage represented by a waveform in FIG. 9(e) is applied to both ends of a pixel connected to the scanning electrode line Y1 and the data electrode line X1. Therefore, when a voltage represented by a solid line waveform shown in FIG. 9(e) is applied to the pixel connected to the scanning electrode line Y1 and the data electrode line X1, the liquid crystal element 71 is turned on, and when a voltage represented by a broken line waveform is applied, the liquid crystal element 71 is turned off.
Generally, the characteristic of the two-terminal element 72 is represented by an I-V (current versus voltage) characteristic that is indicated by a solid line 101 in FIG. 10. Note that the two-terminal element 72, when having a symmetrical characteristic, operates in the same manner irrelevant to the polarity. Therefore, only the case of the positive polarity is illustrated in the figure.
The I-V characteristic of the two-terminal element 72 exhibits a minute current with a high equivalent resistance when the applied voltage is low, while it exhibits an abruptly increased current with a low equivalent resistance when the applied voltage is high.
Accordingly, the two-terminal element 72 having this characteristic can be utilized in a displaying operation. More specifically, a high voltage which causes the two-terminal element 72 to have low resistance is applied to the two-terminal element 72, so that a voltage which turns on the liquid crystal element 71 is applied the liquid crystal element 71. In contrast, a low voltage which causes the two-terminal element 72 to have high resistance is applied to the two-terminal element 72, so that a voltage which turns off the liquid crystal element 71 is applied to the liquid crystal element 71.
Moreover, a voltage which has been applied to the liquid crystal element 71 during a selection period is maintained, since the two-terminal element 72 becomes high-resistive during a non-selection period. Therefore, it is possible to carry out a high-duty driving operation in a display using the two-terminal element 72, compared with a passive-type LCD apparatus.
Furthermore, the LCD apparatus using the two-terminal element 72 can be driven by using the voltage averaging method, whereby a voltage in a waveform shown in FIG. 11 is applied to a pixel, as is the case with a passive-type LCD apparatus. According to the voltage averaging method, a voltage represented by a solid line 111 is applied so that the liquid crystal element 71 is turned on, while a voltage represented by a broken line 112 is applied so that the liquid crystal element 71 is turned off. In short, the liquid crystal element 71 is turned on or off according to the level of the applied voltage during the selection period. Thus, an LCD apparatus driven by the voltage averaging method can ensure high contrast and homogeneity in display by setting a sufficiently big difference between voltages applied for turning on and off during the selection period.
Note that when DC (direct current) components are stored in the liquid crystal element 71, reliability of the liquid crystal element 71 is lowered. In order to avoid this, in general, the polarity of the applied voltage is reversed per frame (or per plural frames, or per plural lines). Therefore, the voltage in the waveform shown in FIG. 11, that is, the voltage applied to the liquid crystal element 71, has the positive and negative polarities alternately at certain intervals. In the following description, the case of the positive polarity is depicted for convenience sake.
When the LCD apparatus using the two-terminal element 72 is driven by the voltage averaging method, there arises a problem as follows: residual images (also referred to as burning) are liable to be produced due to affection of previous display, resulting from that an initial characteristic of the two-terminal element 72 rises.
Such a residual image phenomenon is caused as follows. For example, in an LCD apparatus in normally white mode (in this mode, black is displayed when the liquid crystal element 71 is turned on), as shown in FIG. 12(a), a pattern composed of a white center portion P1 and a black peripheral portion P2 is displayed on a display panel 121. When the display is changed from that having the above pattern to that wherein the whole screen is in gray, that is, in half tone, the pattern previously displayed remains, as shown in FIG. 12(b), causing the display to be inhomogeneous. To be more specific, some difference is caused in the display, between the central portion P1 previously in white and the peripheral portion P2 previously in black, thereby producing a residual image of the previously displayed pattern.
The residual image phenomenon stems from the voltage applying time-dependency of the I-V characteristic of the two-terminal element 72. To be more specific, as shown in FIG. 10, the I-V characteristic of the two-terminal element 72 is shifted from that indicated by a solid curved line 101 to that indicated by a broken curved line 102, as the application of the voltage is continued. Accordingly, as shown in FIG. 13, a V-T (voltage-transmittance) characteristic of the liquid crystal element 71 is also shifted from that indicated by a solid curved line 131 to that indicated by a broken curved line 132. In this case, a voltage whose transmittance is 50%, for example, is shifted from V.sub.50 to V.sub.50' in the figure. Note that the shift amount depends on an applied voltage.
As shown in FIG. 14, the voltage shift amount .DELTA.V (=V.sub.50' -V.sub.50) changes according to a voltage applying period. Moreover, the shift amount .DELTA.V increases as the applied voltage becomes greater. Curved lines 141 and 142 indicate shift amounts .DELTA.V, when an applied voltage in the case of the solid curved line 141 is greater than that in the case of the broken curved line 142.
As is clear from the above description, when the pattern of FIG. 12(a) is displayed, a shift amount .DELTA.V of the peripheral portion P2, to which a higher voltage is applied, is greater than that of the central portion P1. Therefore, when the display is switched from that having the pattern to the monotonous screen in grey which is half tone, namely, the same voltage is respectively applied to the central portion P1 and the peripheral portion P2, the peripheral portion P2 has a higher transmittance compared with the central portion P1 (see FIG. 13). Therefore, the residual image is produced as shown in FIG. 12(b).
Here, there has been proposed a driving method for driving an LCD apparatus, which can eliminate the influence of shift in the I-V characteristic of the two-terminal element on the display state, namely, which can suppress such a residual image phenomenon. The method can be realized by improving the manufacturing process and structure of the two-terminal element.
For example, Japanese Laid-Open Patent Publication No. 8-29748/1996 (Tokukaihei 8-29748) discloses a driving method for an LCD apparatus, wherein the selection period is divided into plural periods. By the method, the residual image phenomenon is reduced by applying a sufficient voltage during the first division of the selection period.
The following description will examine a case where the voltage averaging method, which ensures high contrast and homogeneous display, is adopted in combination with the driving method as described above (hereinafter referred to as dividing-driving method) whereby residual image phenomenon is suppressed by using the selection period divided into plural divisions.
According to the dividing-driving method, a scanning signal shown in FIG. 15(c) and a data signal shown in FIG. 15(d) are produced by selecting voltages out of the liquid crystal driving voltages V.sub.0 through V.sub.5 at six respective levels in accordance with a latch pulse LP shown in FIG. 15(a) and an AC conversion signal M shown in FIG. 15(b). In this case, a difference signal in accordance with a difference between the scanning signal and the data signal, that is, a driving voltage applied to a pixel (the liquid crystal), is either a turning-on voltage or a turning-off voltage in FIG. 15(e), the turning-on and turning-off voltages being represented by the solid line 151 and the broken line 152, respectively. Therefore, there arises a problem that it is difficult to control the driving voltage levels during the selection period, namely, to carry out the control so that a difference between the voltage levels when the liquid crystal is turned on and when it is turned off becomes sufficiently great during the selection period so as to make a high contrast.
With the described dividing-driving method, it is possible to suppress the residual image phenomenon, which stems from the voltage applying time-dependency of the I-V characteristic of the two-terminal element. However, it is difficult to control the driving voltage level during the selection period, and this makes it difficult to combine the dividing-driving method with the voltage averaging method wherein whether the liquid crystal is turned on or off is determined according to the levels of the applied voltage during the selection period.
Moreover, in order to adopt the voltage averaging method to an LCD apparatus of the active matrix driving system driven by the dividing-driving method, it is required to develop new-type drivers for use in such an LCD apparatus, namely, drivers being able to adjust waveforms of voltages outputted therefrom. However, this requires a period of time for developing such drivers, and leads to a rise in the cost of the liquid crystal device.